I. Field of the Invention
This invention relates generally to the field of plasma arc torches, and more particularly to methods and apparatus for treating the collimator employed in the plasma arc torch to reduce the effects of corrosion and thereby extend the service life of the collimator.
II. Discussion of the Prior Art
Plasma arc torches, as known in the prior art, are capable of efficiently converting electrical energy to heat energy producing extremely high temperatures. For example, a plasma arc torch may typically operate in a range as high as from 6000° C. to 7000° C.
Plasma arc torches are known which use water-cooled, reverse polarity, hollow copper electrodes. A gas, such as argon, nitrogen, helium, hydrogen, air, methane or oxygen, is injected through the hollow electrode, ionized and rendered plasma by an electric arc and injected into or integrated with a heating chamber or process.
As is explained in the Hanus, et al. U.S. Pat. No. 5,362,939, plasma arc torches can be made to operate in either of two modes. In a first mode, termed “transferred arc”, a water-cooled rear electrode (anode) applies a high voltage and current to the gas injected into the torch. The material to be heat-treated is made the opposite polarity electrode. As such, the plasma gas passes through a gas vortex generator contained within the torch and out through the central bore of a conductive copper collimator and is made to impinge onto the material serving as the cathode electrode. In the non-transfer arc mode, the arc emanates first from the anode within the torch and reattaches to the cathode at the outlet of the torch. In jumping from the first electrode to the second electrode, the arc extends out beyond the tip of the torch and can be made to impinge upon a workpiece that does not form part of the electrical circuit. Thus, in the non-transferred arc mode, the torch can be used to effectively heat/melt/volatilize non-conductive workpiece materials.
In the case of transfer arc mode torches, the collimator generally comprises a copper holder that screws into the working end of a generally cylindrical torch body in which is contained a rear anode electrode that is electrically-isolated from the collimator. The cylindrical body further contains flow passages for receiving cooling water, routing it through the collimator, and then back through the body of the torch to an outlet port. Likewise, the torch gas has its own passageway to a vortex generator disposed adjacent the central bore of the collimator.
Those readers interested in details of construction of a typical plasma torch are referred to the Hanus, et al. U.S. Pat. No. 5,362,939, the teachings of which are hereby incorporated by reference as if fully set forth herein.
In certain applications of plasma torch technology, the collimator portion of the torch is exposed to corrosive materials. For example, when used in solid waste disposal furnaces to solidify bottom ash and fly ash mixtures into a glass-like mass, chlorine gas is produced from the thermal destruction of plastics. The chlorine can combine with hydrogen to form hydrochloric acid, which can rather rapidly corrode copper surfaces exposed to the acid. It is imperative that the collimator not be corroded to the point where a cooling water channel within the collimator assembly is breached. A stream of water impinging on super-heated surfaces in the furnace can be a serious safety problem and must be avoided. This necessitates frequent shut-down and replacement of the collimators before corrosion reaches the point where the leaking can occur.
The collimator used in transferred arc plasma torches may also experience secondary arcing. In such an arrangement, the collimator is floating in potential and, if the voltage gradient between it and the local plasma potential becomes great enough, a branch of the plasma arc may strike the collimator, pitting and eroding its surface.
It is accordingly a principal object of the present invention to provide a corrosion-resistant barrier on exposed surfaces of the collimator used on plasma torches.
It is a further object of the invention to provide a corrosion barrier that is less subject to cracking due to thermal stresses and/or secondary arcing.